The present invention relates to an aqueous composition for forming zinc phosphate coatings on metal surfaces. When a metal surface is formed, it requires treatment for corrosion resistance and may also require a pre-treatment prior to sealing and/or painting. Zinc phosphate coating solutions are commonly known in the art to pretreat metal surfaces prior to sealing and/or painting. Zinc phosphate coatings isolate corrosion sites. The morphology of the zinc phosphate provides physical anchoring sites for sealing and/or painting solutions which may be applied to the surface in subsequent processing. Zinc phosphate coatings have been successfully used for years to improve the adhesion of paint and other finishes to metal surfaces and to achieve superior corrosion resistance with these finishes.
The drawbacks associated with the application of a zinc phosphate coating solution include the additional process steps which are required in its application. The additional process steps include rinsing and heating the surface after application of the zinc phosphate solution. The rinsing and heating or stoving processes may require a considerable amount of time. The costs associated with these processes include the cost for water rinsing, the disposal of the waste produced after the rinsing process, the equipment and gases and exhaust system used in the heating process, and the processing time used for rinsing and stoving or drying.
The prior art includes solutions and methods for reducing the additional steps associated with the phosphating process and the costs associated with these processes. For example, U.S. Pat. No. 4,659,395 to Sugama et al., provides a process for applying a zinc phosphating solution which does not require subsequent rinsing. However, the prior art of Sugama et al. does require a stoving process to dry the zinc phosphating solution from the surface being treated. The stoving, or other lengthy heating process necessary for curing the surface, is time consuming. This is especially undesirable where the metal surface is produced in a continuous process whereby a metal sheet is continuously painted at speeds of up to 300 feet per minute (or even higher), and must be wound into a coil for storage soon after being produced. In this dynamic processing mode, the metal surface must be treated as it is being produced and before it is rolled into a coil. Therefore, it can be seen that despite advances in the prior art such as those by Sugama et al., a zinc phosphate coating solution which requires neither a rinsing process, nor an extended stoving process for drying, is still lacking in the art.
U.S. patent application Ser. No. 09/275,586, the parent of this application and owned by the assignee hereof, is directed to the use of polyvinyl alcohol, along with zinc oxide, phosphoric acid, and a metal salt in an aqueous solution. Although this composition offers advantages, it has been found that, in some situations, the presence of polyvinyl alcohol tends to cause foaming, solubility problems, and causes the coating to be too tacky.
It is a goal of the present invention to provide a zinc phosphate coating solution which is capable of providing desired adhesion and corrosion resistance qualities by effectively pretreating a metal surface without requiring any additional process operations directed to rinsing or stoving the treated surface.
To achieve these and other objects, and in view of its purposes, the present invention provides a composition and method for forming a zinc phosphate coating on a metal surface. The present invention also provides a method for producing a zinc phosphate solution to be used for pretreating metal surfaces. The zinc phosphate solution is an aqueous solution including water, a zinc source, phosphoric acid, a polyhydric compound, a metal salt, and optionally, an oxidizer. The solution may be formed by mixing water, a zinc source, phosphoric acid, a polyhydric compound, a metal salt, and optionally, an oxidizer. In the solution, at least some of the zinc source and at least some of the phosphoric acid react to form zinc dihydrogen phosphate. Once formed, the zinc dihydrogen phosphate complexes with the metal surface to improve the adhesion of subsequently coated films. It is believed that at least some of the polyhydric compound and at least some of the phosphoric acid react to form a polyhydric phosphate ester upon drying.
The method for forming the phosphate coating involves contacting the metal surface with the solution of the present invention under process conditions such that a zinc phosphate coating is formed on the metal surface. The process sequence for treating the metal surface according to the present invention, includes cleaning the metal surface, rinsing the surface, optionally activating the surface, treating the surface with the zinc phosphate solution of the present invention, mechanically removing the excess solution from the surface, optionally sealing, and painting the surface. The solution of the present invention may be applied to the metal surface by any suitable method. The distinctive advantage of the present invention is that rinsing is not required after application of the phosphate solution as in most phosphate pre-paint pretreatments, nor is an extensive stoving process required.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but not restrictive, of the invention.